1. Field of the Invention
This invention relates to a laser driving apparatus for reading information from a recording medium, and to an optical head apparatus and information processing apparatus using the laser driving apparatus, wherein noise is suppressed by superimposing a radio frequency current on a laser driving current.
2. Description of the Related Art
For optically-readable disk-like recording mediums, optical head apparatuses (so-called optical pickup) including a laser light source are used to read an information signal from, for example, a recordable medium. The laser is driven by a laser driving circuit.
The laser driving circuit employs a photodetector such as a photodiode for receiving a part of laser light and an APC (Automatic Power Control) circuit for controlling a driving current of the laser on the basis of an output signal of the photodetector to control the emitted laser power at a constant level. Further, to suppress noise generated by the laser due to return of the reflection laser light from a disk-like recoding medium (so-called scoop noise), a method of superimposing a radio frequency current on the driving current of the laser is known. For example, Japanese Laid-Open patent application publication No. 2002-335041 discloses this method.
However, in the conventional apparatuses, because the amplitude value of the radio frequency current is fixed, for example, there is a problem in influence on the laser due to change in the temperature characteristic change with time, and influence on a recorded information signal, as follows:
Amplitude of the radio frequency current must be rather large in view of the scoop noise suppressing effect. However, if the amplitude is excessively large, the recorded information signal may be erased. Thus, the amplitude must be set carefully.
FIGS. 13A and 13B are graphical drawings illustrating changes in an optical output of a laser and a drive current with time. In FIG. 13A, the axis of abscissa represents elapsed time, and the axis of ordinate represents an optical output to exemplarily illustrate the change in the optical output with time. In FIG. 13B, the axis of abscissa represents elapsed time, and the axis of ordinate represents a drive current to exemplarily illustrate the change in the drive current with time.
In FIG. 13B, a reference code Ish represents a threshold current, and Iav represents an average current. Emission starts when the drive current exceeds Ish and then, the optical output shows a peak with a delay after the start of emission. That is, the laser output shows a waveform in which a steep pulse emission component is superimposed on a dc (direct current) emission component.
In such laser emission waveform, though the drive current waveform is the same, it is known that the laser emission waveform (for example, a height of a pulse peak) changes in accordance with various laser characteristics such as a relaxation vibration frequency, a differential efficiency, a differential resistance, and the like. Thus, the laser characteristics are also influenced by dispersion among laser devices, temperature characteristics, and change with time. As a result, the amplitude value of the radio frequency current is also influenced from these factors.